1. Field of the Invention
This invention relates to a method and apparatus for monitoring in a fail-safe manner potentiometers used in critical control operations. More particularly, the invention relates to monitoring in a fail safe manner the characteristics of potentiometers where the detected control value from a control potentiometer is a critical safety factor in a control system.
2. Description of Prior Art
Determining the absolute position of valves and/or dampers in combustion systems is a critical safety issue. A system such as a non-flow compensating fuel-air ratio control system controlling a large boiler and using potentiometer settings to control the position of valves and dampers is a system where monitoring of the control potentiometers is a critical life safety factor. Control of actuators for a valve and/or damper in such a combustion system in the past has been accomplished with a motor driven mechanical cam. Such a device has mechanical limits to provide for fail-safe control of the valves and dampers. Due to the mechanical linkages and the limits placed on the mechanical linkages, the fuel-air ratios were never permitted to enter a hazardous range of operation.
In today's environment with microprocessor control and electronic sensing of desired position and electronic feedback to control valves, an electronic failure of a control potentiometer can create a fuel-air ratio in a combustion system that is hazardous. Furthermore, because of the harsh operating environment and because of aging, a control potentiometer's resistance may vary. Also, the resistance at the potentiometer's connector terminals may vary, and the wiper resistance on the potentiometer may change. Any one of these changes or all these changes together can create a situation where an incorrect control signal is provided by the potentiometer monitoring circuit to a controlling microprocessor that controls critical elements such as fuel-air valves. What is needed is a fail-safe monitoring system for potentiometers.